Conventionally, a power screw fastening device is used for fastening a bolt or a screw with predetermined torque. The screw fastening device rotates its shaft continuously so as to fasten a screw and usually performs controlling of turning off the power or disengage a clutch when the torque becomes a certain value.
In various kinds of assembling lines, a worker often fastens a screw with respect to a work on a conveyor belt with a screw fastening device handled by hand.
In this case, it is desirable that the screw fastening device can be operated by one hand from the viewpoint of workability. A screw fastening device for one-hand requires the worker to receive reaction force of fastening a screw by one hand, so there is a problem of increase in the reaction force that is exerted on the worker as a load as the tightening torque increases.
A type of rotating the shaft continuously as described above puts a large load on the worker because the reaction force of the tightening torque is received directly by the worker's hand. In order to reduce this reaction force, an impact type that utilizes rotor inertia of a rotor is used.
However, since the conventional impact type screw fastening device fastens a screw via an impact energy generating mechanism and a socket, accuracy of the tightening torque varies largely when transmission efficiency of them varies. Therefore, it is not suitable for fastening a screw that requires high accuracy. In order to improve the accuracy, there is a method of providing a clutch mechanism to a distal end portion of the shaft so that the clutch is disengaged to control the torque when excessive torque is applied.
From this circumstance, a screw fastening device called an oil pulse wrench is widespread which combines two functions (an impact generating portion and a clutch mechanism) for two tasks of reducing the reaction force and improving the accuracy.
The oil pulse wrench is usually equipped with an oil pulse portion including an air motor that is a driving source and a bypass valve that generates an impulse and can use its oil pressure as a clutch mechanism, which are integrated.
However, the oil pulse wrench has various problems as follows. First, it is troublesome to adjust oil pressure of the bypass valve that works as the clutch mechanism. As the oil pulse generating portion is deteriorated, readjustment and replacement of components should be performed frequently. Accuracy of tightening torque varies depending on oil temperature. Since heat generation by the oil pulse portion is large, countermeasures should be required.
As a device that solves the problems of the conventional oil pulse wrench, a screw fastening device using an electric motor is proposed (see Japanese unexamined patent publication No. 2002-1676.)